The contemporary detectors in the field of leak detection, use is commonly made of detectors comprising a vacuum chamber equipped with a selectively permeable membrane through which only a predetermined gas can pass into the vacuum chamber, with the pressure inside the chamber has been made significantly lower than the outside pressure.
The membranes of known detectors are generally made of quartz or glass with high silica contents, which materials are permeable to helium if they are brought to a suitable temperature, typically at least 300° C. To bring the membrane to such a temperature, an electrical resistor is generally associated with such a membrane. As an alternative to helium, hydrogen can be used as a test gas; however helium is preferable since it is a harmless inert gas, which can be found in very small amounts in the atmosphere, whereby measurement errors are minimised.
The membranes commonly employed have the shape of a capillary with an electrical resistor helically wound around it. A test gas detector, which is equipped with a capillary-shaped membrane, is disclosed, for instance, in the European Patent No. 352,371 assigned to the Assignee of the present invention.
More recently, planar membranes have been developed. Those membranes may have a composite structure, where a thin layer of a material selectively permeable to a test gas, is associated with a supporting layer, impermeable to the test gas and ensuring structural strength. The supporting layer has openings or windows at which the permeable layer is exposed on both faces and through which the test gas can pass into the vacuum chamber. An example of such a membrane is disclosed in the U.S. Pat. No. 3,505,180.
As an alternative, the membranes may have a homogeneous structure and made of a material selectively permeable to test gas. In this case, the membranes have thicker portions ensuring structural stiffness of the membrane and thinner portions locally ensuring high permeability to the test gas. An example of such a membrane is disclosed in the US Patent Publication US 2004/0149131 in assigned the name of the Applicant of the present invention.
Referring to FIG. 1, a prior art device for detecting the presence of test gas is schematically shown. The device, generally shown at 100, comprises a vacuum chamber 102 formed by a hollow cylindrical body 104, one end of which is connected with a suction port of a vacuum pump 106, for instance an ion pump. The other end of chamber 102 is separated from the outside environment by a planar membrane 108 selectively permeable to gases. An electrical resistor, which allows bringing said membrane to a temperature at which it becomes permeable to test gas (e.g. 300° C. for quartz membranes), is deposited on membrane 108, preferably on the face directed towards the outside of chamber 102. The apparatus formed in that manner is placed in the environment to be tested: if test gas is present in the surrounding environment, for instance due to a leak from an enclosure into which said gas was previously introduced, gas enters chamber 102 through membrane 108 and is pumped off therefrom by means of vacuum pump 106. An electronic power supply unit 110 connected with pump 106 is capable of detecting the presence, if any, of test gas inside chamber 102 from the variation of the electrical current absorbed by the pump with respect to vacuum conditions.
The U.S. Pat. No. 5,786,529 discloses a test gas detecting device comprising a chamber, wherein a space is defined for housing a selectively permeable membrane; inlet and outlet ducts for introducing and evacuating the gas in and from the housing are also provided which however does not define any specific path for the gas towards the membrane surface.
Even if use of planar membranes is a significant technical progress over capillary-shaped membranes, planar membrane detectors also are not wholly satisfactory, especially for applications in the technologically most sophisticated fields, where the presence of even minimum amounts of test gas is to be detected. These detectors do not allow attaining a sensitivity level as required by such applications.
Thus, it is an object of the present invention to provide a device and a method for detecting the presence of a test gas, having increased sensitivity with respect to the prior art.
Moreover, as discussed above, the membranes used are permeable to test gas only at high temperatures (for instance, above 300° C. for quartz membranes). Yet, in general, the detector is located in enclosures at ambient temperature, so that the surrounding atmosphere tends to cool the membrane, thus decreasing sensitivity thereof. Therefore, in order to ensure a sufficient sensitivity of the membrane, the power of the heating means associated therewith is to be increased, resulting in increase of the consumption of energy.
Considering that in general, the detectors are battery-powered portable devices, this is a significant disadvantage, since increased energy consumption results in reduced endurance.
Thus, it is another object of the present invention to provide a device and a method for detecting the presence of test gas, which result in reduced energy consumption and, consequently, in increased endurance with respect to the prior art.
Lastly, in the prior art devices, the portion of the membrane surface that is actually permeable to test gas, and therefore utilisable to detect the presence of test gas, is a quite small fraction of the overall surface of the membrane. Test gas molecules coming into contact with the permeable portion, and being consequently detected by the detector, are a correspondingly reduced fraction of the whole of test gas molecules present in the environment surrounding the detector, which is detrimental to the detector sensitivity.
Therefore, it is a further object of the present invention to overcome the above drawback, by providing a device and a method for detecting the presence of test gas, having further increased sensitivity with respect to the prior art.
The above and other objects are achieved by the device and the method for detecting the presence of test gas as claimed in the appended claims.